1. Field of the Invention
The present invention relates to an organic light emitting display, and more particularly, to an organic light emitting display that can prevent degradation of picture quality and a method of manufacturing the organic light emitting display.
2. Description of the Related Art
An organic light emitting display is a self-luminous display that emits light by electrically exciting fluorescent organic compound, and displays an image by driving N×M organic light emitting diodes (OLEDs).
There are two driving methods for the organic light emitting display, that is, a passive matrix (PM) method and an active matrix (AM) method. In the case of the passive matrix method, anode electrodes and cathode electrodes are formed perpendicular to one another and the display is driven by selecting lines. In the case of the active matrix method, transistors and capacitors are connected to pixel electrodes formed of indium tin oxide (ITO) and the display is driven by maintaining a voltage due to the capacitance of the capacitor.
FIG. 1 is a circuit diagram of a unit pixel of a related art AM-type organic light emitting display.
Referring to FIG. 1, a second transistor T2 is connected to an OLED and supplies a driving current for light emission. An amount of the driving current of the second transistor T2 is controlled by a voltage applied through a first transistor T1. The first and second transistors T1 and T2 are PMOS transistors.
A capacitor C1 for maintaining a voltage during a predetermined period is connected between a source and a gate of the second transistor T2. The source of the second transistor T2 is connected to a power supply supplying a voltage VDD. A gate of the first transistor T1 is connected to a gate line GL and a select signal is supplied to the gate thereof. A source of the first transistor T1 is connected to a data line DL and a data voltage is supplied to the source thereof.
An operation of the organic light emitting display of FIG. 1 will be described below.
When the first transistor T1 is turned on in response to the select signal applied to the gate thereof, the data voltage is applied through the data line to the gate of the second transistor T2, that is, to a node X. Thus, the second transistor T2 is turned on and the OLED is driven by the driving current flowing through the turned-on second transistor T2.
The driving current (I) of the second transistor T2 is given by the following Equation 1, which is the same equation as for a general field effect transistor (FET).
                                                        I              =                              K                ⁢                                                                  ⁢                                                      (                                                                  V                        gs                                            -                                              V                        th                                                              )                                    2                                ⁢                                                                  ⁢                where                                                                                        K              =                                                1                  2                                ⁢                μ                ⁢                                                                  ⁢                Cox                ⁢                                                                  ⁢                                  (                                      W                    L                                    )                                                                                        (                  Equation          ⁢                                          ⁢          1                )            
where K is a constant, Vgs is a voltage between the gate and the source of the second transistor T2, Vth is a threshold voltage of the second transistor T2, μ is a mobility, Cox is an oxide capacitance, that is, a capacitance for a unit area of the gate of the second transistor T2, and W and L are respectively a width and a length of the channel of the second transistor T2.
Accordingly, the driving current (I) of the second transistor T2 is controlled by the voltage (Vgs) between the gate and the source of the second transistor T2 and the threshold of the second transistor T2. A brightness of the OLED is controlled by the driving current.
The AM-type organic light emitting display selects a desired pixel using the select signal and drives the OLED by the driving current that flows through the second transistor T2 due to the data voltage.
FIG. 2 is a view illustrating a process of manufacturing the related art organic light emitting display.
Referring to FIG. 2, an amorphous silicon (a-Si) thin film substrate is crystallized into a poly silicon (poly-Si) thin film substrate using a laser power of an Excimer laser. A quality of the poly-Si thin film substrate is determined by various parameters. Specifically, it is very sensitive to the laser power of the Excimer laser. That is, the intensity of the laser power is unstable depending on time and therefore the quality of the poly-Si thin film substrate becomes unstable.
The a-Si thin film substrate is crystallized into the poly-Si thin film substrate by scanning the a-Si thin film substrate with the laser in one direction (that is, scan direction). At this point, the quality of the poly-Si thin film substrate has a non-uniform characteristic in the scan direction and a uniform characteristic in a direction perpendicular to the scan direction.
As a result, if the poly-Si thin film substrate has the non-uniform characteristic, the threshold voltage (Vth) of the drive transistor (e.g., the second transistor T2 in FIG. 1) varies. Thus, all threshold voltages of drive transistors provided at each pixel become different, and driving currents flowing through the drive transistors become different. Consequently, a desired gray scale cannot be obtained.
If the poly-Si thin film substrate crystallized non-uniformly is driven in the manner discussed above, an image having striped patterns is displayed as shown in FIG. 3. This problem is caused by the change in the threshold voltage of each drive transistor in the non-uniformly crystallized substrate of the display.